Transmission of Infectious Agents

Reservoirs (sources of infectious agents) are numerous. Animate reservoirs include people, animals, and insects. Inanimate reservoirs include soil, water, other environmental sources, and medical equipment (e.g., IV solutions, urine collection devices). The host’s body can be a reservoir; pathogens colonize skin and body substances (e.g., feces, sputum, saliva, wound drainage). A person with an active infection or an asymptomatic carrier (one who harbors an infectious agent without active disease) is a reservoir. Examples of community reservoirs include sewage, stagnant or contaminated water, and improperly handled foods.

Bacteria like Neisseria meningitidis can exist in the respiratory tract while causing no illness. If the bacteria invade the bloodstream or cerebrospinal fluid, they become extremely pathogenic. Another example is Enterococcus, which lives as normal flora in the GI system, where it is nonpathogenic and assists in the digestive process. If it enters the bloodstream, Enterococcus can cause disease.

Continued multiplication of a pathogen is sometimes accompanied by toxin production. Toxins are protein molecules released by bacteria to affect host cells at a distant site. Exotoxins are produced and released by certain bacteria into the surrounding environment. Botulism, tetanus, diphtheria, and Escherichia coli 0157:H7–related systemic diseases are attributed to exotoxins. Endotoxins are produced in the cell walls of certain bacteria and released only with cell lysis. For example, typhoid and meningococcal diseases are caused by endotoxins.

The patient’s immune status plays a large role in determining risk for infection. Congenital abnormalities, as well as acquired health problems (e.g., renal failure, steroid dependence, cancer, acquired immune deficiency syndrome [AIDS]), can result in numerous immunologic deficiencies. Depression of the immune system may make the host more susceptible to infection or cripple the ability to combat organisms that have gained entry.

Medical and surgical interventions may impair normal immune response. Steroid therapy, chemotherapy, and anti-rejection drugs increase the risk for infection. Medical devices (e.g., intravascular or urinary catheters, endotracheal tubes, synthetic implants) may also interfere with normal host defense mechanisms. Surgery, trauma, radiation therapy, and burns result in nonintact skin. The body’s skin is one of the best barriers or defenses against infection. When this barrier is broken, infection often results.

Microorganisms may enter the body in a variety of ways, including the respiratory tract, GI tract, genitourinary tract, skin and mucous membranes, and bloodstream.

Pathogens may enter the body through the respiratory tract. Microbes in droplets are sprayed into the air when people with infected oral or nasal tissues talk, cough, or sneeze. A susceptible host then inhales droplets, and pathogens localize in the lungs or are distributed via the lymphatic system or bloodstream to other areas of the body. Microorganisms that enter the body by the respiratory tract and produce distant infection include influenza virus, Mycobacterium tuberculosis, and Streptococcus pneumoniae.

Other pathogens enter the body through the GI tract. Some stay there and produce disease (e.g., Shigella causing self-limited disease). Others invade the GI tract to produce local and distant infection (e.g., Salmonella enteritidis). Some produce limited GI symptoms, causing systemic infection (e.g., Salmonella typhi) or profound involvement of other organs (e.g., hepatitis A virus). Millions of foodborne illness cases occur each year in the United States. This type of illness results in many hospitalizations and deaths.

Microorganisms also enter through the genitourinary tract. Urinary tract infection (UTI) is one of the most common health care–associated infections (HAIs). More than half of patients in adult intensive care units (ICUs) have urinary catheters in place. Indwelling urinary catheters are a primary cause of UTIs. The use of invasive catheters has decreased in long-term care settings because older adults are very susceptible to infections.

Although intact skin is the best barrier to prevent most infections, some pathogens such as Treponema pallidum can enter the body through intact skin or mucous membranes. Most enter through breaks in these normally effective surface barriers. Sometimes a medical procedure creates a break in cutaneous or mucocutaneous barriers, as in catheter-acquired bacteremia (bacteria in the bloodstream) and surgical-site infections (SSIs). Fragile skin of older patients and of those receiving prolonged steroid therapy increases infection risk.

Contact transmission is the usual mode of transmission of most infections. Many infections are spread by direct or indirect contact. With direct contact, the source and host have physical contact. Microorganisms are transferred directly from skin to skin or from mucous membrane to mucous membrane. Often called person-to-person transmission, direct contact is best illustrated by the spread of the “common cold.”

Indirect contact transmission involves the transfer of microorganisms from a source to a host by passive transfer from a contaminated object. Contaminated articles or hands may be sources of infection. For example, patient-care devices like glucometers and electronic thermometers may transmit pathogens if they are contaminated with blood or body fluids. Uniforms, laboratory coats, and isolation gowns used as part of personal protective equipment may be contaminated as well.

Physiologic Defenses for Infection

Intact skin forms the first and most important physical barrier to the entry of microorganisms. In addition to providing a mechanical barrier, the skin’s slightly acidic pH (resulting from breakdown of lipids into fatty acids), together with normal skin flora, creates an unfriendly environment for many bacteria.

Mucous membranes’ mucociliary action provides some mechanical protection against pathogenic invasion. More important, however, mucous membranes are bathed in secretions that inactivate many microorganisms. Lysozymes, which dissolve the cell walls of some bacteria, are present in large quantities in many body secretions, particularly in tears and nasal mucus.

Other body systems provide natural barriers to infection. For instance, the healthy respiratory tract clears about 90% of all inhaled material by upper airway filtration, humidification, mucociliary transport, and coughing. Peristaltic action mechanically empties the GI tract of pathogenic organisms. Stomach acid, intestinal secretions, pancreatic enzymes, and bile, together with the competition from normal bowel flora, provide an environment that protects the GI tract from invasion by harmful organisms. In the genitourinary tract, the flushing action of urine eliminates pathogenic organisms. The low pH of urine also maintains a sterile environment, although some microorganisms, such as E. coli, thrive in an acid medium.

Phagocytosis occurs when a foreign substance evades the first-line mechanical barriers and enters the body. Various leukocyte types function differently in the immune reaction, but neutrophils bear primary responsibility for phagocytosis. This process of engulfing, ingesting, killing, and disposing of an invading organism is an essential mechanism in host defense. Phagocytic dysfunction dramatically increases a patient’s risk for infection.

Infection Control in Inpatient Health Care Agencies

The infection control program of a hospital is usually coordinated and implemented by a health care professional certified in infection control (CIC) who has clinical and administrative experience. The Centers for Disease Control and Prevention (CDC) recommends one person with CIC credentials for every 100 occupied acute care beds. Long-term care facilities may not have a practitioner who is dedicated to infection control. However, every facility must designate a health care professional to be responsible for coordinating and implementing an infection prevention and control program.

Infection Control in Community-Based Settings

Ambulatory and home health care are the fastest growing segments of the health care system. Infection remains the most common cause of death for dialysis patients. Data on infections in other ambulatory settings are lacking because the CDC has only recently recommended surveillance systems and prevention guidelines. Little information is available about acquired infections in home health settings because data are not systematically collected, surveillance programs are not established, and best practices for infection prevention and control do not exist.

Methods of Infection Control

Effective handwashing includes wetting, soaping, lathering, applying friction under running water for at least 15 seconds, rinsing, and adequate drying. Friction is essential to remove skin oils and to disperse transient bacteria and soil from hand surfaces. Performing adequate handwashing takes time that health care workers (HCWs) may not feel they have. Handwashing can also cause dry skin, and therefore hand moisturizers are essential to maintain good hand health and hygiene.

Alcohol-based hand rubs (ABHRs) allow care providers to spend less time seeking out sinks and more time delivering care. However, these hand rubs have their limitations.

The CDC recommends using antiseptic solutions such as chlorhexidine, povidone-iodine, or parachlorometaxylenol (PCMX) for handwashing in caring for patients who are at high risk for infection (e.g., those who are immunocompromised). The use of these solutions is also recommended after caring for patients who are infected with multidrug-resistant or other virulent organisms.

Most health care settings in the United States and Canada have switched from latex to non-latex gloves. The U.S. National Institute for Occupational Safety and Health (NIOSH) issued a public warning about potential allergic reactions to those exposed to latex in gloves and other medical products. Reactions include rashes, nasal or eye symptoms, asthma, and (rarely) shock. People with latex allergy usually have an allergy to foods such as bananas, kiwis, and avocados. Health care workers (HCWs) have not been as strict with wearing gloves as they should because of poor fit or skin dryness, irritation, and dermatitis. One possible solution to dry skin is the use of aloe vera–coated gloves or moisturizers such as Eucerin or AmLactin products.